Terraforming refers to 'turning another planet into a second terra'. Marsforming would be 'turning the planet into a second Mars', which wouldn't be particularly useful, since we can't live on Mars anyway.

The joke they're making is that since we are in the process of turning the Earth into a second Mars, we should be calling what we are doing now marsforming.

i think you'll find that venus is the place with the runaway greenhouse effect, not mars!

Whoosh... Sorry

We're going towards Venus rather than Mars, though. So "veneraforming".

The combining form would be marti-. (Compare "Martian".)